/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package javax.servlet.jsp.tagext;

import javax.servlet.jsp.JspException;


/**
 * The BodyTag interface extends IterationTag by defining additional methods that let a tag handler
 * manipulate the content of evaluating its body.
 * <p>
 * It is the responsibility of the tag handler to manipulate the body content. For example the tag
 * handler may take the body content, convert it into a String using the bodyContent.getString
 * method and then use it. Or the tag handler may take the body content and write it out into its
 * enclosing JspWriter using the bodyContent.writeOut method.
 * <p>
 * A tag handler that implements BodyTag is treated as one that implements IterationTag, except that
 * the doStartTag method can return SKIP_BODY, EVAL_BODY_INCLUDE or EVAL_BODY_BUFFERED.
 * <p>
 * If EVAL_BODY_INCLUDE is returned, then evaluation happens as in IterationTag.
 * <p>
 * If EVAL_BODY_BUFFERED is returned, then a BodyContent object will be created (by code generated
 * by the JSP compiler) to capture the body evaluation. The code generated by the JSP compiler
 * obtains the BodyContent object by calling the pushBody method of the current pageContext, which
 * additionally has the effect of saving the previous out value. The page compiler returns this
 * object by calling the popBody method of the PageContext class; the call also restores the value
 * of out.
 * <p>
 * The interface provides one new property with a setter method and one new action method.
 * <p>
 * <B>Properties</B>
 * <p>
 * There is a new property: bodyContent, to contain the BodyContent object, where the JSP Page
 * implementation object will place the evaluation (and reevaluation, if appropriate) of the body.
 * The setter method (setBodyContent) will only be invoked if doStartTag() returns
 * EVAL_BODY_BUFFERED and the corresponding action element does not have an empty body.
 * <p>
 * <B>Methods</B>
 * <p>
 * In addition to the setter method for the bodyContent property, there is a new action method:
 * doInitBody(), which is invoked right after setBodyContent() and before the body evaluation. This
 * method is only invoked if doStartTag() returns EVAL_BODY_BUFFERED.
 * <p>
 * <B>Lifecycle</B>
 * <p>
 * Lifecycle details are described by the transition diagram below. Exceptions that are thrown
 * during the computation of doStartTag(), setBodyContent(), doInitBody(), BODY, doAfterBody()
 * interrupt the execution sequence and are propagated up the stack, unless the tag handler
 * implements the TryCatchFinally interface; see that interface for details.
 * <p>
 * <IMG src="doc-files/BodyTagProtocol.gif" alt="Lifecycle Details Transition Diagram for BodyTag">
 * <p>
 * <B>Empty and Non-Empty Action</B>
 * <p>
 * If the TagLibraryDescriptor file indicates that the action must always have an empty element
 * body, by an &lt;body-content&gt; entry of "empty", then the doStartTag() method must return
 * SKIP_BODY. Otherwise, the doStartTag() method may return SKIP_BODY, EVAL_BODY_INCLUDE, or
 * EVAL_BODY_BUFFERED.
 * <p>
 * Note that which methods are invoked after the doStartTag() depends on both the return value and
 * on if the custom action element is empty or not in the JSP page, not how it's declared in the
 * TLD.
 * <p>
 * If SKIP_BODY is returned the body is not evaluated, and doEndTag() is invoked.
 * <p>
 * If EVAL_BODY_INCLUDE is returned, and the custom action element is not empty, setBodyContent() is
 * not invoked, doInitBody() is not invoked, the body is evaluated and "passed through" to the
 * current out, doAfterBody() is invoked and then, after zero or more iterations, doEndTag() is
 * invoked. If the custom action element is empty, only doStart() and doEndTag() are invoked.
 * <p>
 * If EVAL_BODY_BUFFERED is returned, and the custom action element is not empty, setBodyContent()
 * is invoked, doInitBody() is invoked, the body is evaluated, doAfterBody() is invoked, and then,
 * after zero or more iterations, doEndTag() is invoked. If the custom action element is empty, only
 * doStart() and doEndTag() are invoked.
 */
public interface BodyTag extends IterationTag {

    /**
     * Deprecated constant that has the same value as EVAL_BODY_BUFFERED and EVAL_BODY_AGAIN. This
     * name has been marked as deprecated to encourage the use of the two different terms, which are
     * much more descriptive.
     *
     * @deprecated As of Java JSP API 1.2, use BodyTag.EVAL_BODY_BUFFERED or
     * IterationTag.EVAL_BODY_AGAIN.
     */
    @SuppressWarnings("dep-ann")
    // TCK signature test fails with annotation
    public static final int EVAL_BODY_TAG = 2;

    /**
     * Request the creation of new buffer, a BodyContent on which to evaluate the body of this tag.
     * Returned from doStartTag when it implements BodyTag. This is an illegal return value for
     * doStartTag when the class does not implement BodyTag.
     */
    public static final int EVAL_BODY_BUFFERED = 2;

    /**
     * Set the bodyContent property. This method is invoked by the JSP page implementation object at
     * most once per action invocation. This method will be invoked before doInitBody. This method
     * will not be invoked for empty tags or for non-empty tags whose doStartTag() method returns
     * SKIP_BODY or EVAL_BODY_INCLUDE.
     * <p>
     * When setBodyContent is invoked, the value of the implicit object out has already been changed
     * in the pageContext object. The BodyContent object passed will have not data on it but may have
     * been reused (and cleared) from some previous invocation.
     * <p>
     * The BodyContent object is available and with the appropriate content until after the invocation
     * of the doEndTag method, at which case it may be reused.
     *
     * @param b the BodyContent
     * @see #doInitBody
     * @see #doAfterBody
     */
    void setBodyContent(BodyContent b);

    /**
     * Prepare for evaluation of the body. This method is invoked by the JSP page implementation
     * object after setBodyContent and before the first time the body is to be evaluated. This method
     * will not be invoked for empty tags or for non-empty tags whose doStartTag() method returns
     * SKIP_BODY or EVAL_BODY_INCLUDE.
     * <p>
     * The JSP container will resynchronize the values of any AT_BEGIN and NESTED variables (defined
     * by the associated TagExtraInfo or TLD) after the invocation of doInitBody().
     *
     * @throws JspException if an error occurred while processing this tag
     * @see #doAfterBody
     */
    void doInitBody() throws JspException;
}
